15 April 2017

an LDL160 transfer with nine retransmissions

this is a copy of an 10 x LDL160 transfer, heard on 8327.0KHz/USB, with up to nine retransmissions of the same 'Citadel' encrypted data packet (Fig. 1). Since the LDL forwards are followed by ACKs from the receiver station, although they are faintly visible, the nine retransmissions could be due to repeat requests.

Fig. 1

The analysis of the last 64 bits of each BW3 bursts shows a fixed structure in the Sequence Number field: if the reps had been in the datagram passed to the LDL protocol then the sequence number would have been incremented, moreover the EOM and SOM fileds (bits 16,15) are both to 1 in all the packets meaning that this is the only packet in the sent datagram (Fig. 2)

Fig. 2

As said, since the ARQ ACK mechanism of xDL protocols, it could be that the nine retransmissions are requested by the receiver (bad channel conditions at receiver side?) but it could also be a technique to improve the reliability of the transmission: transmitting the same information more than once (each time encoded using alternate FEC codes) during a particular link increase the probability that user data will be received correctly.